Ensure DockedStackDividerController sets a dim for one stack at a time.

Only set a dim for one stack. If the dimmed stack is already set and
there's a request for dim for another stack, clear the old stack's dim.
This ensures that no old dims are left around with no way to clear them.

Test: Dock an app that can open an IME. Open the IME and then hit home.
There should be no dim layers around when in minimized state.
Fixes: 70894010

Change-Id: I7ddbd4c133acf069f811cf1ead0dd79b0c4118a9